This invention relates to the preparation of metal oxide superconducting ceramics and more particularly to the use of new processing additives for preparing the sintered superconductors. Since the discovery of metal oxide superconducting ceramics, considerable effort has been directed to the development of compositions with high Tc and improved performance with emphasis being placed on processing techniques. In these ceramic compositions, it has been important that the grain contact areas have minimal contamination with nonsuperconducting phases or other materials which would adversely affect the passage of current.
One of the important metal-oxide superconductors has the composition YBa.sub.2 Cu.sub.3 O.sub.7-.delta. where .delta. may vary from about 0.1-0.4 with the composition being commonly identified as "123". Preparation of the composition involves the mixing of Y.sub.2 O.sub.3, CuO and BaCO.sub.3, calcining of the mixture to form YBa.sub.2 Cu.sub.3 O.sub.7-.delta., grinding to reduce particle size, forming into a "green" body and sintering to form the final product. Usually, a solvent and binder are included as processing additives after calcining. The solvent is usually selected to have the property of being volatilized at temperatures below about 100.degree. C. with the binder remaining until an ignition temperature of about 500.degree. C. is reached with the sintering subsequently being carried out at about 850.degree.-975.degree. C. An illustration of a common solvent-binder combination is methanol and polyvinyl alcohol.
Since the binder is removed by burning, some residue is formed on the grains. While the extent of the problem has not been measured, the formation of residue between the grains will increase the resistance between grains and limit the current density of the superconductor.
Another problem which may occur in the processing is the potential for an adverse effect of any moisture or alcohol on the composition or components. In the preparation of the superconducting ceramic, it is of considerable importance to maximize the percentage of the desired superconductor and avoid or limit the levels of materials which are not superconducting.
Accordingly, one object of this invention is the preparation of a superconducting composition with limited residue formed between interconnecting grains of the superconductor. Another object of the invention is the preparation of a superconducting composition in which the presence of water or other hydroxy-containing compound is minimized. These and other objects of the invention will become apparent from the following description.